Homodyne network analysis processes have an advantage over heterodyne processes in that they can be carried out utilizing a single monochromatic high-frequency source. To recover the information with respect to magnitude and phase of the four-pole parameters to be determined into the low-frequency range in which they can be evaluated, a portion of the source signal must have a low-frequency modulation imposed thereon. The modulated source signal is then transmitted to the measured object and after the transmission of the reflection from the measured object, the reflected signal is mixed with an unmodulated portion of the source signal to effect the transfer to the low-frequency range. This modulation method determines whether the low-frequency information obtained can yield the desired four-pole parameter.
It is known, e.g. from IEEE Trans. Microwave Theory Techn. Vol. MTT-13 1965, page 371 ff, J. S. Jaffe, R. C. Mackey, that a homodyne network analyzer can be constructed using phase modulators which effect a saw-tooth or step-function modulation with a fixed-phase variation of two (single sideband phase) modulators. This method has, however, disadvantages and sensitivities which prevent it from achieving high measurement accuracy over a wide frequency range at reasonable cost. In other words, any attempt to increase the precision of this earlier system results in a substantial increase in the cost of the circuit and the complexity thereof.
Single sideband modulators when used as phase modulators ensure a sufficient sideband suppression only upon precise preservation of the modulation form and variation degree and give rise to parasitic amplitude modulations resulting in measurement errors. Furthermore, an increase in precision is not possible through the use of simple means.